Semiconductor devices require a series of fabrication processing steps in the production of a device. The steps can include the formation of layers of oxide, interconnect material, and thin film materials, and diffusion into a layer or substrate. Each layer requires the steps of, masking, etching away of unwanted material, and subsequent cleaning. These steps requires time, materials, and processing equipment to perform. Further, these steps can damage the device and thus reduce the device yield from a substrate. Thus, if multiple types of semiconductor devices are fabricated on the same substrate, the requirement for any additional layers increase the overall cost of producing each device.
FIG. 1 illustrates a standard memory cell 10 for flash memory, a type of reprogrammable non-volatile memory. Flash memory operates on the principle of using a MOS transistor with a floating gate 14 to selectively store a charge. The operation of the channel 17 in the MOSFET will change upon whether a charge is stored on the floating gate. The effect on the conductivity of the MOSFET channel is detectible and thus the state of the memory cell, either a logical 1 or 0, is detectable by read logic (not shown). The MOSFET transistor can be of the type of either N-channel or P-channel. FIG. 1 shows an N-channel device. For the purposes of clarity, structures required to isolate one memory cell from another memory cell or other devices are not included in the description. Further, the description excludes the required circuitry for reading the programed state of the memory device.
It is often desirable to fabricate different types of semiconductor devices on a single substrate. Such a combined device fabrication can reduces packaging costs, produces an overall smaller combined device footprint, decreases signal delay times between components and thus faster operating speeds, and reduces power requirements through reduced chip interfaces and the associated signal drivers. However, different types of semiconductor devices such as logic and flash memory require a different number of processing steps. For example, a conventional flash memory typically requires the formation of an additional layer from which a floating gate is formed and thus requires the additional steps and costs of masking and oxidation for the extra gate formation. Thus, the benefits are obvious of producing reprogrammable non-volatile memory and other semiconductor devices and on the same semiconductor substrate that can be fabricated the same processing steps and/or processing layers as other types of semiconductor devices. One drawback of using the same oxide layers for both a non-volatile memory device and another semiconductor device (logic devices, analog devices, microcontrollers), is that the oxide layer thickness optimal for one type of device is not likely to be optimal for forming a non-volatile memory device with a high program/erase cycle capability. The thinner oxides use for logic devices or microcontrollers can be damaged by the program/erase cycle and these memory are only good for less than a hundred program/erase cycles. However, there is a class of applications where a non-volatile memory device with a limited program/erase cycle capability is still useful. Such applications might include setting an encryption key or programing an identification number. What is needed is a device and fabrication method that forms a reprogrammable non-volatile memory without requiring additional layers or processing steps beyond what is required by an addition type of semiconductor device, such as logic, analog, micro-controller/processor, or graphics processing devices, formed on the same substrate.